In the related art, when a mesh-type optical network is configured, a redundancy mechanism of a line or a path is employed to improve reliability of the network. In devices that include a terminal device such as Synchronous Optical NETwork (SONET) or Synchronous Digital Hierarchy (SDH), which performs a photoelectric conversion on a data signal, if the redundancy configuration of the photoelectric conversion is used, this may generally increase a cost and a signal delay. For this reason, in the devices, the redundancy configuration using the optical switch is preferable as long as optical characteristics allow.
Meanwhile, in the redundancy configuration based on the optical switch, interception of a signal according to a switching operation of the optical switch is also propagated to the downstream side and an optical switch of the downstream side is also switched interlocking with the switching of the optical switch. As a result, the fluttering phenomenon of unnecessary switching being frequently generated in plural optical switches is generated. As a method of suppressing the fluttering phenomenon from being generated, a used technique is to desensitize failure detection of the optical switch located at the downstream side using a switch control circuit having a timer function illustrated in FIG. 17 in the optical switch, that is, a technique of delaying light input break detection is used. As a result, switching timing of the optical switches that are connected in multi-steps is adjusted to prevent the fluttering phenomenon from being generated.
Next, an example of the case where the fluttering phenomenon is prevented by the optical switches having the timer function will be described using FIG. 18. FIG. 18 illustrates a first example of a network where the optical switches having the timer function are configured in multi-steps.
In the first example of the network configuration illustrated in FIG. 18, an A station (relay station), a B station (relay station), and a C station (terminal station) are provided and an optical signal is transmitted to each station through paths of two systems of a 0 system and a 1 system. The A station has an optical switch #1 and a coupler #1. The B station has an optical switch #2 where a hold-off time (delay time) of 50 msec is set and a coupler #2, and outputs an optical signal input from the A station to the C station. The C station has an optical switch #3 where a hold-off time (delay time) of 100 msec is set, and receives an optical signal input from the B station and terminates the optical paths of the two systems.
In this configuration, the optical switch #1 of the A station detects loss of light (LOL) of the 0 system path. In this case, transmission of the signal with respect to the downstream side (side of the B station) is stopped (shutdown) in both systems and the LOL is detected in both the B station and the C station. Then, the optical switch #1 of the A station switches a selector (SEL) from the 0 system to the 1 system, and the failure of the network is restored.
Meanwhile, the optical switch #2 of the B station collects a state of the 1 system with a polling period of the LOL and detects restoration of the LOL. At this time, the delay time of 50 msec is set to the optical switch #2 of the B station. Since the delay time of 50 msec does not pass from the detection of the LOL, SEL switching is not executed. Similar to the above case, the optical switch #3 of the C station collects a state of the 1 system with a polling period of the LOL and detects restoration of the LOL. At this time, in the optical switch #3 of the C station where the delay time of 100 msec is set, since the delay time of 100 msec does not pass from the detection of the LOL, SEL switching is not executed.
Then, after the delay time of 50 msec passes, the optical switch #2 of the B station detects that both the 0 system and the 1 system are normal and determines that switching of the SEL is not done. Likewise, after the delay time of 100 msec passes, the optical switch #3 of the C station detects that both the 0 system and the 1 system are normal and determines that the switching of the SEL is not done. That is, when the failure is detected in the A station of the upstream side, the optical switch of the A station immediately executes switching. Meanwhile, the optical switches of the B and C stations of the downstream side do not execute the switching, when restoration of the failure is detected before the delay time passes.
As described above, the timer that can set the delay time is provided in the optical switch of the downstream side, thereby monitoring an input of the optical signal for the delay time, instead of switching the optical switch whenever interception or supply of the optical signal is detected. After the delay time passes, when the optical signal is not input, the failure is restored by executing the switching by the optical switch and the fluttering phenomenon based on the switching of the plural optical switches is prevented from being generated.
However, in the related art, a long delay time to be set to the optical switch of the downstream side, and a long time is taken to restore the failure. Specifically, since switching timing of the optical switch becomes timing after the delay time passes, the optical switch of the downstream side where the long delay time is set is switched after a long time passes from the generation of the failure. That is, a long time is taken to restore a failure of the entire network.
For example, the case where the failure is generated in the B station in the network of FIG. 18 will be described. Specifically, it is assumed that the optical switch #2 of the B station detects the LOL in the 0 system. In this case, the optical switch #2 of the B station that is connected to the 0 system does not execute the switching of the SEL, because the delay time of 50 msec is set even though the LOL is detected in the 0 system and the 1 system is normal. That is, even though the optical switch #2 of the B station detects the LOL, the optical switch does not immediately execute the switching. For this reason, the optical switch #3 of the C station is affected by the failure of the B station and detects the LOL of both systems with the polling period of the LOL.
After the delay time of 50 msec passes from the generation of the failure, that is, the delay time of the optical switch #2 of the B station passes, the optical switch #2 detects that the LOL is detected in the 0 system and the 1 system is normal and switches the SEL from the 0 system to the 1 system. Meanwhile, the optical switch #3 of the C station collects a state of the 1 system with the polling period of the LOL and detects that the LOL is restored. Since this point of time is a point of time before the delay time of 100 msec passes, the optical switch #3 of the C station does not execute the switching of the SEL. After the delay time of 100 msec passes, the optical switch #3 of the C station detects that both the 0 system and the 1 system are normal by the switching of the B station, and determines that the switching of the SEL is not done.
As such, when the failure is detected in the B station, quick switching of the SEL is not executed and the restoration is delayed by 50 msec as compared with the case where the failure is detected in the A station. Even when the LOL is detected by the optical switch #3 of the C station, the switching of the SEL is not executed until the delay time of 100 msec passes. For this reason, the restoration is delayed by 50 msec as compared with the example of the B station. Even in the case of a second example of the network illustrated in FIG. 19, in the optical switch of the C station where the delay time is set, even though a failure is detected in the C station and the LOL is detected, the switching of the SEL is not executed until the delay time passes. As a result, the restoration is delayed by the delay time.
That is, when the optical switch where the delay time is set detects the failure, the switching of the SEL is not executed until the delay time passes. For this reason, the restoration is delayed by the delay time. As such, when the optical switches that have the switch control circuit with the timer function are used, the fluttering phenomenon of the optical switches when the failure is detected can be prevented, but a long failure restoration time may be taken according to the failure part.